Measuring based on a vibrating sensor of angular velocity has proved to be a method of measuring angular velocity having a simple concept and being reliable. In a vibrating sensor of angular velocity, a certain known primary motion is produced and it is maintained in the sensor. The motion desired to be measured with the sensor is then detected as a deviation of the primary motion.
Central features required of sensors of angular velocity are resistance to shaking and impact. Particularly in demanding applications, such as e.g. driving control systems in the car industry, these requirements are extremely tight. Even a sharp blow, like for instance an external impact caused by a stone, or the vibration caused by a car stereo should not influence the output of the sensor of angular velocity.
The principle of operation of vibrating sensors of angular velocity most often used is the so called tuning fork principle. In the tuning fork principle the primary motion is a vibration of two linear resonators vibrating in opposite phase on a common axis. An external angular velocity affecting the sensor in a direction perpendicular to the direction of motion of the resonators causes Coriolis forces influencing the masses in opposite directions.
A Coriolis force proportional to the angular velocity is detected either directly from the masses, or the masses are connected on the same rotational axis, whereby the detection motion is angular vibration in the direction of the angular velocity axis. The angular vibration to be detected is, however, susceptible to external mechanical interference.
Inevitably, from impact events and vibrations, instances of angular acceleration, due to vibrations of the material and the substructure, are generated also to the detection axis of the sensor of angular velocity. Then the motion of the detection resonator is disturbed and deviations are caused to the output signal of the sensor of angular velocity, particularly when the frequency of the interference is close to the operating frequency of the sensor.
Several sensor solutions of prior art have been presented in order to compensate for interference signals. Such a sensor has often been made by using so called differential detection, whereby a sensor structure can be achieved, which is considerably less insensitive to external mechanical interference. One such prior art sensor solution, for example, is described in the U.S. Pat. No. 6,705,164, wherein two seismic masses, positioned alongside each other, are linearly vibrating in opposite phases in the surface plane and in the same direction. Thus, the detection resonator axis is common for both masses and perpendicular to the primary motion.
In differential detection, a common axis of motion for the centers of gravity of the detection resonators provides optimal insensitivity to external mechanical disturbance. Differential detection with two masses cancels external linear accelerations and, in the special case of a common detection axis, also instances of angular acceleration, since the sensitivity to angular acceleration of a traditional tuning fork gyro is proportional to the distance between the detection resonators axes.
Thus, this implementation of a sensor of angular velocity according to the so called reversed tuning fork principle operates significantly more reliably in shaking conditions in comparison with e.g. a traditional tuning fork. However, the greatest weakness of the reversed tuning fork principle is the primary motion's susceptibility to external disturbances.
In the structure according to the U.S. Pat. No. 6,705,164 mentioned above, the coupled linear resonators are more easily mobile in a common phase than in opposite phase. Then, the linear acceleration along the primary axis can fairly easily disturb the operation of the sensor, since the masses are physically displaced, although the differential detection of them to a major extent mitigates the thereby caused signal.
A better solution than the differential detection for compensation of mechanical disturbances is the so called dual differential detection. Such a prior art sensor solution is i.a. described in the U.S. Pat. No. 6,122,961, wherein, in FIG. 3, a dual differential tuning fork structure is presented, which structure also includes the reverse tuning fork operating principle with parallel masses presented above.
In the U.S. Pat. No. 6,122,961, four masses are coupled together into a dual differential tuning fork structure, in which two pairs of differential masses according to the tuning fork are vibrating in opposite phase in parallel. This prior art structure thus contains two opposite phase reversed tuning forks.
The dual differential tuning fork is, in fact, the most reliable angular velocity sensor structure based on linear motion in one plane. Its greatest weakness is, however, an extremely complicated and bulky spring structure. The excitation structures are also difficult to design and they also take a lot of space.
Another significant challenge in designing a good sensor of angular velocity is the capacitive crosstalk of electrostatic drive, particularly into Coriolis detection. The drive signal of the primary motion is in phase with the speed of the masses, and then the crosstalk signal is most often observed precisely in phase with the Coriolis signal. This causes a disturbance in the value of the zero point of the sensor dependent on the amplitude of the drive signal, which causes an error dependent on i.a. the temperature.
A conventional solution to this problem is the so called carrier detection. In carrier detection an AC voltage, i.e. a carrier wave, is applied across a capacitance to be measured. In this arrangement, the variable capacitance is made to modulate the amplitude of the output signal of a preamplifier, and a voltage dependent on the capacitance is obtained by demodulating with the carrier. Then the interference signal caused by capacitive crosstalk will be modulated away from the signal band at the carrier frequency.
However, carrier detection complicates the already quite complicated electronics of the sensor of angular velocity, which then contributes to increasing cost and size of the electronics and impairs its reliability. It is actually worthwhile to design the capacitive sensor element in such a way, that crosstalk of the drive signal caused by stray capacitances will cancel out as thoroughly as possible in differential Coriolis detection.
The objective of the invention is, indeed, to achieve a structure of a vibrating sensor of angular velocity suitable for a small size, and being resistant to interference, by means of which angular velocity perpendicular to the surface plane can be measured in a reliable manner.